Test for InterlockedCompareExchange64 in the configure script.

mingw.org doesn't have this function but mingw-w64 does.

This commit is licensed under MIT/X11.
Fix undeclared variables in exceptions-amd64.c on win64.

This commit is licensed under MIT/X11.
Merge pull request #763 from madewokherd/mingwbuild

Fix the mingw-w64 build